An overload relay is used to protect electrical equipment, such as, for example, motors, controllers and branch-circuit conductors, from current overload. The overload relay is connected between a power source and the electrical equipment. When an overload condition exists, the overload relay opens electrical contacts (e.g., normally closed (NC) contacts) to interrupt power to the equipment via a contactor or other circuit interrupter. The overload relay can also include other electrical contacts (e.g., normally open (NO) contacts), which are closed to turn on an alarm in response to the overload condition.
There are different types of overload relays, such as a thermal overload relay, melting alloy overload relay, bimetallic overload relay, and magnetic current relay. An overload relay can include a sensing element to detect a current overload condition (e.g., a high current condition or over current condition) and an actuating element to actuate a trip mechanism which opens the electrical contacts, such as normally closed (NC) contacts, when a current overload condition is detected by the sensing element. Some overload relays use a heating coil as the sensing element and a bimetallic strip as the actuating element for each current phase. The bimetallic strip has the heating coil wound directly thereon. The heating coil is a conductor which is connected to receive current (e.g., one phase of the current) that flows to the electrical equipment. In operation, the heating coil is heated by current flow therethrough. The bimetallic strip is configured to deflect and actuate the trip mechanism to open the electrical contacts when the bimetallic strip is heated by the heating coil at or above a threshold temperature which reflects a current overload condition, e.g., a high current condition.
Accordingly, these types of overload relays require at least two or more parts for the sensing and actuating elements (e.g., a heating coil and a bimetallic strip), thereby increasing complexity of assembly, potential frictional failure due to the contact of two parts, and overall costs. Such overload relays also require a substantial amount of materials for the sensing and actuating elements and require substantial current calibration.